Image forming apparatus

Abstract

An image forming apparatus includes a photosensitive member, a charging roller, a voltage source, a current detecting member, and a setting portion configured to set the DC voltage applied to the charging roller in an image forming period on the basis of a detection result of the detecting member when a DC voltage less than a discharge start voltage is applied from the voltage source to the charging roller in a period other than the image forming period, wherein the setting portion sets the DC voltage so that an absolute value of the DC voltage when an absolute value of the detected current is a first value is larger than an absolute value of the DC voltage when the absolute value of the detected current is a second value smaller than the first value.

What is claimed is: 
     
       1. An image forming apparatus comprising:
 a photosensitive member on which an electrostatic latent image is formed; 
 a charging roller in contact with said photosensitive member and configured to electrically charge said photosensitive member; 
 a voltage source configured to apply only a DC voltage to said charging roller; 
 a detecting member configured to detect a current flowing from said charging roller into said photosensitive member; and 
 a setting portion configured to set the DC voltage applied to said charging roller in an image forming period on the basis of a detection result of said detecting member when the DC voltage less than a discharge start voltage is applied from said voltage source to said charging roller in a period other than the image forming period, wherein said setting portion sets the DC voltage so that an absolute value of the DC voltage when an absolute value of the detected current is a first value is greater than an absolute value of the DC voltage when the absolute value of the detected current is a second value less than the first value. 
 
     
     
       2. An image forming apparatus according to  claim 1 , further comprising a storing portion configured to store information on said photosensitive member including at least one of information on a thickness of said photosensitive member, information on a cumulative rotation time of said photosensitive member and information on use hysteresis,
 wherein said setting portion sets the DC voltage on the basis of the information on the photosensitive member. 
 
     
     
       3. An image forming apparatus according to  claim 1 , further comprising an environment detecting portion configured to detect environment information including at least ambient temperature and ambient humidity of said photosensitive member,
 wherein said setting portion sets the DC voltage on the basis of the detected environment information. 
 
     
     
       4. An image forming apparatus according to  claim 1 , further comprising:
 an exposure member configured to expose a surface of said photosensitive member to light on the basis of image information to form the electrostatic latent image including an image portion and a white background portion; and 
 a developing device configured to deposit toner only on the image portion of the electrostatic latent image formed on said photosensitive member, 
 wherein said setting portion sets an amount of exposure of the white background portion to light when the absolute value of the detected current is the first value to be greater than that when the absolute value of the detected current is the second value less than the first value.